The present invention relates to a device for vaporization injection into a gas chromatography analysis device.
Several methods are known of injecting and vaporizing samples into a gas chromatography analysis device, in which the samples formed by the substance to be analyzed and a solvent, are injected by means of syringes equipped with a needle into the vaporization chambers generally in the form of heated cylinders, in which the said samples vaporize and then pass into the capillary column for analysis. The type of injection can be split or splitless, depending on whether the sample is passed only in the part through the column (xe2x80x9csplit modexe2x80x9d) or in its entirety (xe2x80x9csplitless modexe2x80x9d).
In each case, it is important that the sample vaporizes in an optimum manner and in its entirety within the vaporization chamber, in a position above the column entrance where it is temporarily held before being transferred to the column for analysis.
In the known vaporization methods, it is provided to use long needles in order to inject the sample into the centre or at the base of the vaporization chamber. In particular, for the splitless method, in accordance with the current technique, the needles need to reach a certain point so that the vaporization centre occurs a little above the column entrance. This is to avoid sample loss occurring towards the top of the chamber.
In these systems, the vaporization chamber is completely heated to a temperature above the vaporization temperature of the sample to be analyzed, and the sample injected through the needle is vaporized immediately.
Two alternative techniques are used for vaporizing the samples. The first commonly used technique foresees a so-called flash vaporization of the sample within the chamber, with preheating and pre-vaporization of the sample within the needle. The time spent by the needle within the chamber increases the temperature of the same and this causes a violent evaporation of the solvent present within the sample, which causes the liquid to be expelled from the needle at a high pressure; on leaving the needle, the liquid is broken up into tiny droplets forming an aerosol so that the droplets are slowed down and can therefore be immediately vaporized. The drawbacks of this technique are connected to the fragility of the unavoidably long needles which have to be used for this type of injection, and to the discrimination phenomenon, mainly because a high boiling part of the sample may remain inside the needle.
As a result, conventional vaporization chambers normally are shorter than ideal dimensions, with much smaller internal volumes than desirable. Frequently it is also possible to register sample losses through the head of the chamber and the purge of the septum due to the xe2x80x9coverflowxe2x80x9d phenomenon. The second alternative technique, so-called xe2x80x9ccold-fast injectionxe2x80x9d, foresees the insertion of the needle and the expulsion of the liquid within fractions of a second to eliminate or substantially reduce the phenomenon of sample heating in the needle and the consequent discrimination.
However, this technique presents other drawbacks, for example it may only be carried out using automatic samplers and not manually, as it is not possible for an operator to manually carry out all the movements necessary for this technique which are required to be carried out in a time of the order of 500 milliseconds or less. As a consequence, the vaporization of the sample in the injector is substantially different according to whether the injection was carried out manually or automatically, and the analysis results gathered according to the two methods are not homogenous.
The present invention originates from the discovery made by one of the inventors according to which, if the injection of the sample is carried out without using the xe2x80x9cthermal sprayxe2x80x9d technique, or if the injection is carried out avoiding the vaporization of the sample in the needle, the ejected liquid forms a liquid jet or band, which travels a long way across the chamber in a practically unaltered form without touching the chamber walls, as close to these walls it is repelled by the vaporization of small quantities of solvent.
Based on these observations, it has been possible to create a vaporization injection device which eliminates the drawbacks of the known techniques, and allows a highly reliable analysis even of samples of considerable volume. It has been ascertained, and this constitutes the base of the present invention, that the injection samples without vaporization within the needle, allows to overcome the previous limitations due to the fact that the sample had to be injected close to the base of the vaporizing chamber or close to a stop and vaporization means for the liquid, such as a pack or another obstacle. Therefore, the invention concerns a vaporization injection device of the type without vaporization within the needle, in which the distance from the point or free end of the needle to the stop and vaporization means is much longer than that provided by the prior technique, and in particular greater than 55 mm and preferably greater than 80 mm.
The minimum value quoted above has been set in order to allow the vaporization injection also of large quantities of samples in a reliable manner, without recording xe2x80x9coverflowxe2x80x9d losses as described above.
In order to obtain a sample delivery as a liquid band it is fundamental that the inside of the needle is cold, or at a temperature below the boiling point of the solvent at the pressure within the same needle, and that for the entire duration of the injection.
A good method to obtain this is to use a very short needle, which can now be used as the foreseen distance from the point of the needle to the stop means and vaporization of the liquid is longer than above. Any minimal vaporization before the injection in such a short needle does not substantially influence the emission of the sample as a jet or band.
However, it is possible to take other precautions to guarantee this type of emission under all operating conditions. These precautions, which may be applied singularly or together, essentially consist of:
reduction of the internal diameter of the needle, to reduce the quantity of sample within the same needle before emission starts,
maintenance of the injection head, and therefore of the needle, at a relatively low temperaturexe2x80x94by cooling or simply ambient thermal exchangexe2x80x94in a manner that the temperature of the needle remains below that of vaporization of the solvent before and during injection,
protection of the needle with a thermal insulating covering.
forming the needle in a thermal insulating material.
The invention will be now described in a detailed manner with reference to particular embodiments of the same, illustrated as examples in the annexed drawings.